In mice daily testing by maximal electroshock (MES) and pentylenetetrazole infusion (PTZ) 2 h after an i.p. injection of 25 mg/kg penobarbital (PB) was performed on days 1, 2, 4 through 8. The first twitch was delayed as compared to NaCl controls (50.4 +/- 4.8 s; 36.6 +/- 3.6 s; n = 53) on day 2, 4, 6 through 8. There was no tolerance of the PTZ induced tonic extension with Pb-treatment. In contrast, MES-tested mice developed tolerance, at day 5 the number of protected animals fell to about 50%. In view of the constant phenobarbital levels in plasma, a functional tolerance was assumed...